The Fort McKinley Historic District comprises a concentration of 44 buildings all but one of which are masonry construction. Located at the eastern end of Great Diamond Island in Casco Bay, the district includes only the residential and support buildings in the center of the fort. The largest number of structures are concentrated around an elliptical parade ground. Additional structures are situated on curvilinear roads to the north, east and south of these buildings.
Fort McKinley Historic District is significant as the largest of five military complexes which made up Portland’s harbor defenses in the early part of this century. Constructed substantially in 1903-1910, this fort provided Maine’s principal city with one of the best defended harbors in the country. Its construction on Great Diamond Island was part of the general upgrading of the American military initiated by Theodore Roosevelt and his Secretary of War, Elihu Root. The district includes the central core of residential and support buildings which housed the island garrison. These buildings represent the most intact concentration of its kind in the State of Maine.
Erected near the resort community of Great Diamond Island, Fort McKinley was the centerpiece to a complex of five forts which provided the defense of Portland Harbor. Its construction followed the general push for military preparedness initiated by President Theodore Roosevelt after the Spanish-American War.
Designed for a garrison of 700 men, the fort included a large group of brick residential structures designed in the Colonial Revival style. These buildings housed both officers and enlisted men. Also included were smaller brick buildings erected to provide support facilities. There was a fire station, a power station, a school, a hospital, and a bakery, as well as other utilitarian structures.
Fort McKinley continued to play an active part in Portland’s harbor defenses until its deactivation by the government in the 1940’s. Following deactivation, the structures deteriorated as a result of their abandoned state.
In the 1980's the complex was purchased by private developers and the buildings were rehabilitated and the complex was redeveloped as a private resort community.
The Fort McKinley Historic District was listed in the National Register of Historic Places in 1985 and became a local historic district in 1990.